In a cold-forming process which results in work-hardening of a material, successive steps in forming the material become more and more difficult, requiring a greater degree of force. Cold-forming of a material as a result of forming threads by the turning of a screw into a workpiece, requires a successive increase in effort at moving the necessary amount of material.
The prior art shows a number of different approaches to attempt to reduce driving torque. U.S. Pat. No. 3,426,642 issued to Phipard and entitled SELF-TAPPING SCREWS WITH THREAD-FORMING PROJECTIONS is an example of an attempt to reduce driving torque. The Phipard patent employs a plurality of projections or protuberances which are arranged "over at least the work-hardening end portion (emphasis added)." A drawback of having protuberances on a plurality of pitches such as in Phipard results in increased friction, and not in an optimized reduced driving torque.
The Phipard patent further does not solve the problem of a greater degree of force encountered by succeeding protuberances under work-hardening principles, as the required force necessary to move successive protuberances through the workpiece becomes greater and greater, even though the protuberances are the same size. The prior art further does not snow any screws which include protuberances or projections which show any special geometry to reduce the force necessary to promote deformation of the workpiece.
Accordingly, a need has arisen for a screw which takes into full account the considerations of work-hardening, such that the screw is specifically configured to move successive parts of the screw through the workpiece without requiring a greater degree of force. Where protuberances or projections are employed in an attempt to reduce driving friction, generally, a need has arisen for self-tapping screws or taps which promote better deformation of the workpiece.